Substrains of S. mansoni selected for varying infectivity to vector snails have been tested for their ability to induce immunity in mice. Two strains derived from the same patient and designated PRT-3 and PRC-3 induced markedly different degrees of resistance to reinfection after a bisexual first infection. The F-1 cross between these strains also produced high resistance, comparable to that induced by the "immunogenic" PRT-3 strain. C57BL/KsJ mice with unisexual S. mansoni infections generally were about 40% resistant to challenge infections; however, the PRC-3 strain induced no resistance after unisexual infection. The PRT-3 and PRC-3 strains have been selected for 5 generations on the basis of their ability to induce resistance, but no apparent selection of higher or lower resistance traits has been achieved.